1. Field of the Invention
This invention relates to a thermosensitive recording method using a sublimation-type thermosensitive image transfer recording medium.
2. Discussion of Background
Recently the demand for full color printers is increasing year by year. Typical recording methods for full color printers now available include the electrophotographic method, the ink-jet method, and the thermosensitive image transfer method. Of these methods, the thermosensitive image transfer method is most widely employed because of its advantages over the other methods in that maintenance is easy and operation is noiseless.
In the thermosensitive image transfer recording; method, a solidified color ink sheet and a receiving sheet are employed, and a color ink is transferred imagewise from the ink sheet to the receiving sheet due to the thermal fusion or the sublimation of the ink, under the application of thermal energy by laser beams or a thermal head which is controlled by electric signals.
Thus, the thermosensitive image transfer recording method can be roughly classified into two types, a thermal fusing image transfer type and a sublimation image transfer type. The sublimation image transfer type is advantageous over the thermal fusing type in that halftone can be obtained without difficulty and image gradation can be controlled as desired. These benefits exist because a sublimable dye is in principle sublimated in the form of independent molecules in such an amount as to correspond to the amount of thermal energy applied thereto, for instance, through a thermal head. Therefore, the sublimation image transfer type is considered the most suitable for color printers.
The sublimation image transfer recording method, however, has a shortcoming in that its running cost is high, because in this image transfer method, a yellow ink sheet, a magenta ink sheet, a cyan ink sheet and, if necessary, a black ink sheet, are employed in order to obtain a full-color image, with selective application of thermal energy to each ink sheet, and discarded after the recording, even though large unused portions remain on each ink sheet.
In order to eliminate this shortcoming, a multiple printing recording method has been proposed, in which an ink sheet is used repeatedly.
The multiple printing recording method includes an equal-speed mode method and an n-time-speed mode method. In the former method, an ink sheet and an image receiving sheet are moved at the same speed when images are recorded. In the latter method, on the other hand, the running speed of the image receiving sheet is made n (n&gt;1) times the running speed of the ink sheet when images are printed, and the ink sheet is shifted little by little in such a manner that the first used portion and the second used portion are overlapped each other. It is therefore a matter of course that a larger value of "n" contributes to higher cost reduction.
Since a non-used portion of the ink sheet is provided together with a used portion when images are printed, the n-time-speed mode method can minimize the scatter of the amount of a residual ink. In the equal-speed mode method, on the other hand, a used portion of the ink sheet is merely used repeatedly. Therefore, the n-time-speed mode method is advantageous over the equal-speed mode method as reported in the Journal of the Institute of Electronics and Communication Engineers, Vol. J70-C, No. 11, pages 1537-1544 (1987).
In the sublimation-type thermosensitive image transfer recording method, the sublimation and evaporation reaction is basically a reaction of zero order. Therefore, even if the ink layer of the ink sheet is provided with a sufficient amount a dye for multiple printing, the ink sheet cannot be used for multiple printing even when the n-time-speed mode method is employed. The printed image density significantly decreases as the value of "n" increases, in other words, as the relative speed of the recording medium decreases, particularly in high image density areas.